Simulation of Oxygen Delignification in an Existing Bleaching Plant for Waste Minimization

I. Dogan, G.A. GuruzFaculty Member, Prof. Dr., TR

Keywords:

Oxygen delignification, OCEHDED simulation, effluent reduction

Abstract:

Simulation of Oxygen Delignification in an Existing Bleaching Plant for Waste Minimization Dr. smail Do an, TUBITAK, Tunus Cad., No:80, Ankara, TURKEY, fax: +90(312)4274305, phone: +90(312)4671801, ismail.dogan@tubitak.gov.tr Prof. Dr. Güniz Gürüz, METU, Chemical Engineering Department, 06531, Ankara, TURKEY, fax: +90(312)2101264, phone: +90(312)2102634, guniz@metu.edu.tr Pulp bleaching has traditionally been carried out with chlorine chemicals, notably elemental chlorine, to react with the lignin remaining on the fibers after cooking which produce a large number of different hazardous chlorinated organics. Bleach plant effluents are normally not recovered or recycled, since the organic content is comparatively low, and they can be corrosive due to chlorides. Therefore, the improvement of bleach plant effluents has focused on reducing the concentration and quantity of effluents by a number of methods which can be grouped as pre-bleaching, bleaching and post-bleaching modifications. The pre-bleaching methods are oxygen delignification and modified cooking. The bleaching modifications include elemental chlorine free (ECF) and total chlorine free (TCF) bleaching, operational analysis and optimization of process variables. Post-bleaching modifications are the waste water treatment systems. In this study, an oxygen delignification unit was added to an existing steady state model of a sixstage chlorination - extraction - hypochlorination - chlorine dioxide - extraction - chlorine dioxide (CEHDED) bleach plant which has been previously simulated (1). Moreover, the effect of adding an oxygen delignification unit on the total effluent coming out from the six washers in the bleach plant was investigated by applying different case studies. A source code was written to simulate the oxygen delignification unit and the whole bleach plant. The total dissolved solids (TDS) is used throughout this study as an indication of the amount of effluents coming out from six bleaching stages. Total dissolved solids represents the lignin and carbohydrates that has reacted with the bleaching chemicals and the unreacted chemicals in the streams. Although TDS is not a routine mill measurement, it is easily tracked with process simulation because TDS mass is conserved. Each stage of the sequence is composed of chemical additions and mixing, reaction in a retention tower, and washing. Unit operation models were formulated using mass balances on liquor, fibres, chromophores, chemicals and dissolved solids. In order to compare the simulation results with the mill data, a pulp and paper mill bleaching unit located on the southern part of Turkey was chosen. Good agreement between the simulation results and the mill data was achieved. In the first part of the case studies, the effect of process variables in oxygen delignification unit which were oxygen pressure, alkali concentration and temperature on the TDS was investigated and optimized. When the oxygen delignification tower process variables are optimized, there is a 17.96% decrease in the total amount of pollutants with an output brightness of 92.95. In the second part of the study, by applying the optimum variables in oxygen delignification unit the effect of retention tower operating temperature, residence time, consistency, bleaching liquor flow rate and the wash water flow rates in each stage of the bleach plant on the TDS were studied. Among the parameters investigated; the highest reduction in the total amount of dissolved solids, with a value of 22%, was achieved by optimizing the bleaching chemicals used in each stage. (1) . Do an and G. Gürüz.